Mixture-compressing, spark-ignited reciprocating-piston internal combustion engines are customarily operated by means of a throttle control. As opposed to the full load state, in the partial-load range of the engine the amount of intake fuel and air will be greatly reduced, although the fuel-air ratio remains approximately the same. For any given engine speed (RPM) this results in a decreased final compression pressure as the throttling increases (i.e., as the load decreases) so that in accordance with the known thermodynamic relationships, the thermal efficiency at part load drops considerably in comparison with full load, resulting in an increase of the specific consumption of fuel in the engine.
A method is known for substantially improving the thermal efficiency at part load wherein the geometric compression ratio is adjusted such that the effective compression ratio, and thus the final compression pressure, is maintained approximately constant. The effective compression ratio can be defined according to the following relationship: ##EQU1## where V.sub.S is the volume of fuel-air mixture taken into the cylinder during the induction stroke and V.sub.C is the compression chamber volume at the end of the compression stroke (at compression top dead center). An internal combustion engine having both throttle and compression control may be operated to achieve a fuel efficiency even better than that of diesel engines, and at the same time result in reduced emission pollutants.
An internal combustion engine having combined throttle and compression control is described in U.S. Pat. No. 3,961,607 in which each main piston is associated with an auxiliary piston sliding in an auxiliary cylinder, the auxiliary piston being actuable by a secondary crankshaft driven by the main crankshaft. The compression chambers of the main and the auxiliary pistons are in communication at all times, and by control of the phase position the total compression volume, consisting of the sum of the volumes of the main and auxiliary piston chambers when the main piston is at compression top dead center, can be varied as a function of the operating parameters of the engine.
In the engine disclosed in the U.S. Pat. No. 3,961,607, improved performance is achieved by driving the auxiliary piston at a higher stroke frequency than that of the main piston, so that the auxiliary piston performs several complete strokes during a single stroke of the main piston. This must necessarily lead to very high piston speeds of the auxiliary piston, resulting in considerably increased wear and tear and, in the case of an intermediate transmission such as the planetary transmission proposed in aforementioned patent, in considerable additional noise.